Fibre Optic Source Emits Paired Light at Two Wavelengths

Researchers are developing increasingly sophisticated sources of entangled photons for applications in quantum communication and sensing. Keshav Kapoor, Dong Beom Kim, and Kriti Shetty, all from the University of Illinois Urbana-Champaign, alongside Virginia O Lorenz and colleagues at the same institution, have demonstrated a novel photon-pair source fabricated within commercially available optical fibre. This device generates paired photons at both near-infrared and telecommunication wavelengths, specifically separated by 700nm, offering a significant advantage by minimising Raman noise and achieving a high coincidence-to-accidental ratio at room temperature. The source’s unique ability to produce spectrally distinct, phase-matched processes, combined with its use of readily available materials and potential for multiplexing, positions it as a promising candidate for practical implementation in future quantum networks.

Scientists have created a compact device capable of generating entangled photons at wavelengths vital for future optical networks, offering a pathway towards secure data transmission. By utilising standard optical fibre, the technology promises a cost-effective and readily deployable solution. This work details a compact device, built using commercially available optical fibre, capable of generating photon pairs with a substantial wavelength separation of 700nm.

The high degree of ‘non-degeneracy’ is particularly advantageous, effectively minimising interference from unwanted noise and allowing for room-temperature operation. Beyond simply creating photon pairs, this research demonstrates the ability to produce two distinct, spectrally separate processes simultaneously, each with unique spatial characteristics. Once a cornerstone of theoretical physics, quantum communication is now edging closer to practical implementation, and efficient photon sources are essential.

Dual stimulated four-wave mixing generates E and S band photon pairs

Joint spectral intensity measurements reveal two distinct photon-pair generation processes, one centred at 830nm and the other at 850nm, originating from separate stimulated four-wave mixing interactions within the polarization-maintaining fibre. Detailed spectral analysis shows that process 1 generates telecommunication-wavelength photons falling within the E-band, while process 2 produces photons in the S-band.

The observed diagonal streaks in the joint spectral intensities are attributed to chirp induced by the pump pulse propagating through the fibre. Singles count rates were carefully measured to characterise the source’s output, revealing process 1 exhibited a higher photon flux than process 2, a distinction confirmed by subsequent rate measurements. Coincidence measurements yielded a g(2)(0) value of 0.007 for process 1, confirming that exactly one photon is emitted at a time.

This low value indicates a highly non-classical light source. Process 2 also demonstrated strong photon pairing, though at a slightly lower rate. Further analysis focused on the coincidence-to-accidental ratio (CAR), a key metric for assessing the quality of the photon pairs. The source achieved a CAR exceeding 10, even while operating at room temperature, signifying minimal background noise and efficient pair generation despite the absence of cooling.

The detectors used, superconducting nanowire single-photon detectors with over 90% efficiency for telecommunication wavelengths and avalanche photodiodes with 45% efficiency at NIR wavelengths, contributed to the precision of these measurements. These results demonstrate the source’s suitability for deployment in quantum networks requiring high-rate, spectrally distinct photon pairs.

Birefringent fibre phase-matching for nondegenerate photon-pair generation

A commercially available optical fibre serves as the core of this photon-pair source, engineered to generate entangled photons at both telecommunication (1500nm) and near-infrared (830nm) wavelengths. This setup exploits spontaneous four-wave mixing, where intense light propagating through the fibre creates correlated photon pairs with a substantial wavelength separation of 700nm.

Careful selection of fibre characteristics was essential; the birefringent nature of the fibre enabled phase-matching of the generated photon pairs. Two distinct phase-matched processes occur simultaneously, each contributing to the generation of a photon pair at a specific wavelength. To minimise unwanted spectral overlap, the system was designed to produce highly nondegenerate pairs, detuning them from potential Raman noise sources.

This non-degeneracy is a key advantage, allowing for high coincidence-to-accidental ratios even at room temperature, simplifying practical implementation. Further refinement involved controlling the spatial characteristics of the emitted photons, with near-infrared photons exhibiting distinct transverse spatial modes and telecommunication photons confined to a single fundamental spatial mode.

This precise control was achieved through careful fibre design and manipulation of the input beam, ensuring efficient coupling into single-mode fibres for downstream applications. Stimulated emission tomography was employed to fully characterise the spatio-spectral quantum state of the generated photon pairs.

Room-temperature photon pairing in standard fibre enables advanced quantum networks

Scientists have devised a method for generating paired photons across crucial telecommunications wavelengths using readily available optical fibre, bypassing the need for complex or custom-built components. For years, creating dependable sources of these entangled or paired photons has demanded painstaking alignment of exotic materials and precise temperature control, limiting their use beyond the laboratory.

Now, a room-temperature source built from standard fibre offers a pathway towards practical devices. Generating photons at both near-infrared and standard telecommunication wavelengths, separated by a substantial spectral distance, opens possibilities for advanced networking, underpinning secure communication protocols and distributed quantum computing architectures.

While previous attempts often suffered from unwanted noise or weak signals, this design minimises these issues through careful control of the light’s spatial characteristics and a clever use of fibre properties. Still, challenges remain. Scaling up the photon pair production rate is essential for many applications, and current rates are not yet at the levels needed for high-bandwidth quantum networks.

Furthermore, understanding the long-term stability of the source requires further investigation. Beyond this, researchers will need to address the integration of these sources with existing fibre infrastructure. Once these hurdles are overcome, the potential is considerable, and a surge in efforts to build compact, all-fibre quantum systems is expected.

Other groups are exploring alternative materials and fabrication techniques, and this work provides a valuable benchmark against which to measure progress. The future likely holds hybrid approaches, combining the best aspects of different technologies to create a truly practical and scalable quantum internet.